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Title: Single and multi-layered thin film oxides for potential fuel cell applications
Author: Cook, Stuart N.
ISNI:       0000 0004 2728 9764
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2012
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Recently there has been a great level of interest in the effect of interfaces in oxide ionic conductors with a view to eventual application in solid oxide fuel cells (SOFCs). Enhancements in electrical conductivity of one half to eight orders of magnitude have been reported in simplified thin film and multi-layered heterostructure systems. Often this is reported to be enhanced oxygen ion conduction with little supporting evidence. The aim of this work is to investigate these reports and develop an understanding of the underlying mechanisms. Several series of samples were investigated to achieve this goal. The first, designed to emulate an anomalous result from literature with alternating samarium doped ceria (SDC) and undoped ceria layers, featured an increasing total number of layers of equal individual thickness, and thus a constant interfacial density. This system featured no enhancement in conductivity and exhibited a level of tracer diffusion comparable with that in bulk SDC. Electron energy loss spectroscopy (EELS) studies, however, revealed a significant level of Ce III in the undoped layers. The second and third series used similar materials but tested the hypothesis proposed in many works, that conductivity enhancement was related to tensile strain in the conducting material at the heterointerfaces. The second, manipulating the strain at the interface by varying the dopant (Nd, Sm, Y) in films with alternating doped and undoped ceria layers and a range of interfacial densities. This series exhibited minimal change in conductivity with strain or interfacial density. The third series replaced the doped ceria with yttria-stabilised zirconia (YSZ) in order to achieve a higher level of tensile strain. This again featured minimal change in conductivity. Tracer diffusion and secondary ion mass spectrometry (SIMS) studies suggested that the undoped ceria layers featured vacancy-rich regions, close to the interfaces, possibly with compensating Ce III. The final series of multilayers comprised alternating praseodymium nickel copper gallate (Pr1.91 Ni0.71 Cu0.24 Ga0.05 O4) and SDC layers which exhibited a high level of conductivity and evidence of reduced levels of p-type conduction with decreasing SDC layer thickness, suggesting enhanced ionic conductivity. Oxygen tracer studies revealed, however, that the dominant charge carrier was not oxygen. Finally a study of the effect of dislocations in ionic conductors was performed on deformed single crystal YSZ. Impedance measurements revealed a small enhancement in conductivity in the orientation parallel to the dislocation cores however diffusion measurements showed a change that could be negated by the consideration of the inherent errors.
Supervisor: Kilner, John Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral